1. Field of the Invention
The present invention relates to a flyback converter system, and more particularly, to a flyback converter system selectively using a primary feedback signal or a secondary side feedback signal.
2. Description of Prior Art
The power supply for electronic products generally needs electrical isolation between a power input end and a power output end. The electrical isolation can be realized by, for example, an isolation transformer. In a flyback converter, the output power is adjusted by controlling a series of pulses applied to the primary winding of the converter. More specifically, the turning-on time of the pulse is prolonged when the output power increases, while the turning-on time of the pulse is shortened when the output power decreases.
A conventional current mode flyback converter is shown in FIG. 1. In the conventional flyback system, output regulation is provided through the optocoupler (OPTO). In one popular implementation, the output voltage Vout is divided into a voltage Vdiv through a resistor network. Vdiv controls the shunt regulator TL431 which generates a current proportional to the difference of Vdiv and an internal regulated voltage in TL431, typically at 2.5V. The current generated will be converted into a feedback voltage (FB) through the optocoupler, OPTO. The PWM controller uses FB signal to control the on time of the switch Q such that proper voltage regulation is achieved.
In FIG. 1, Waux is the auxiliary winding which provides the power needed for the PWM controller. It can also be used to provide valuable information of the system such as input line voltage Vin and the voltage on the secondary winding Vy. For example:
Vx=Vy when Q is off
Vx=−Vin when Q is on
One continuous effort for flyback converter is to save as much power as possible in both normal load and light load or no-load conditions. The problem for light load or no-load condition is more prominent as any current consumed in the converter system itself constitutes a major part of the total power consumption. For a typical AC/DC adapter system, for example, vast majority of the time is spent in no load or light load condition. Hence any power saving in these condition will become significant overall power saving. One of the major components of current consumption in a flyback system is the output regulation circuit described above. A typical shunt regulator or other regulators consumes a few mille-amperes. Tens of mille-watts of power are needed for the regulation circuit. While the regulation circuit in regular load condition plays very important role to assure the output voltage is within the desired operating range, it is an important task to reduce its current consumption in light load condition.
Therefore, it is desirable to provide a flyback converter system with accurate output regulation and fast dynamic response during normal-load (full-load) condition, and with less power consumption during light-load or no-load condition.